User manual

Appendix B: Audio Considerations B-1
Moseley SL9003Q 602-12016 Revision G
Appendix B: Audio Considerations
B.1 Units of Audio Measurement
B.1.1 Why dBm?
In the early years of broadcasting and professional audio, audio circuits with matched
terminations and maximum power transfer were the common case in studios and for audio
transmission lines between facilities. Console and line amplifier output impedances,
implemented with vacuum tube and transformer technology, were typically 600 Ohms.
Equipment input impedances, again usually transformer-matched, were also typically 600
Ohms. Maximum power transfer takes place when the source and load impedances are
matched. For such systems, the dBm unit (dB relative to one milliwatt) was appropriate since it
is a power unit.
B.1.2 Audio Meters
However, actual power-measuring instruments are extremely rare in audio. Audio meters and
distortions analyzers are voltmeters, measuring voltage across their input terminals. They do
not know the power level, current value, nor source impedance across which they are
measuring, Since the audio industry had “grown up” with 600 Ohm power-transfer systems in
common use, audio test instrument manufacturers typically calibrated their voltmeters for this
situation. Most audio test instruments and systems manufactured before approximately 1985
used only Volts and the dBm unit on their meter scales and switch labels. The dBm unit was
calibrated with the assumption that the meter would always be connected across a 600 Ohm
circuit when measuring dBm. Since the voltage across a 600 Ohm resistor is 0.7746 Volts
when one milliwatt is being dissipated in that resistor, the meters were actually calibrated for a
zero “dBm” indication with 0.7746 Volts applied. But, they were not measuring power; change
the circuit impedance, and the meter is incorrect.
B.1.3 Voltage-Based Systems
Modern audio equipment normally has output impedances much lower than input impedances.
Output impedance values from zero up to 50 Ohms are typical, and input impedances of 10
kilohms are typical. Such equipment, connected together, transfers negligible power due to the
large impedance mismatch. However, nearly all the source voltage is transferred. As noted
earlier, a 10 kilohm load reduces the open-circuit voltage from a 50 Ohm source by only 0.5%,
or 0.05 dB. Thus, modern systems typically operate on a voltage transfer basis and the dBm,
as a power unit, is not appropriate. A proper unit for voltage-based systems is the dBu (dB
relative to 0.7746 Volts). The dBu is a voltage unit and requires no assumptions about current,
power, or impedance. Those older audio meters calibrated in “dBm” are really dBu meters.